Microwave dichroic plates are employed with Cassegrain antennas to reflect signals in one frequency band and to pass signals in a second frequency band. One prior art dichroic plate, as illustrated in FIG. 1, employs an array of crossed dipoles 11 having centers 12 that lie along rows and columns 15 and 16, respectively parallel to X and Y axes of a Cartesian coordinate system. Each of crossed dipoles 11 includes a pair of arms 17 and 18 that are crossed with each other to have orthogonally directed longitudinal axes. Each of arms 17 and 18 has approximately the same length, approximately 1/2 the wavelength of a cut-off frequency of the array, i.e., the frequency that separates the frequency bands thare are reflected and passed by the plate. The longitudinal axes of arms 17 and 18 are inclined at 45.degree. relative to the X and Y axes. Because centers 12 of adjacent ones of elements 11 are either parallel with the X and Y axes or a line between adjacent elements has a 45.degree. angle relative to the X and Y axes, a line between the centers of the adjacent elements has a zero component relative to either the X or Y axis, or the X and Y components of the line are the same.
A plate constructed in accordance with FIG. 1 has the disadvantage of a relatively large separation between centers 12 of adjacent elements 11. As the spacing between the dipole arms of adjacent elements approaches zero, the separation between the centers of the adjacent elements approaches .sqroot.2/3 (approximately 0.471) of the wavelength of the plate cut-off frequency. It is desired to minimize the spacing between the centers of adjacent elements to minimize grating lobes of the high frequency electromagnetic energy propagated through the plate. Grating lobes reduce the energy passed through the dichroic plate along an antenna boresight axis, and decrease the bandwidth that can be passed through the plate. It has also been demonstrated both empirically and theoretically that higher order modes exist in the slots for asymmetrical excitations, and these higher order modes combine with the dominant, low order, mode and the losses so produced also limit the available band width. The energy reduction and higher order modes are particularly significant if the plate, or a portion of the plate, is inclined relative to the boresight axis of a high frequency feed, such as employed in a Cassegrain antenna. Because dichroic plates frequently have a hyperbolic reflecting surface in a Cassegrain antenna, it is apparent that a considerable portion of the plate is inclined relative to the boresight axis and all off-axis impinging energy impinges the plate at angles of incidences greater than zero.